You say retain a BT landline , do you currently have a ‘landline’ broadband service from BT ( regardless of not yet being on Digital Voice ) ?
A landline only service from BT is currently is £30.20 on a PAYG basis or £48.20 with the Unlimited Minutes call plan , that’s probably not much cheaper than broadband and telephone, you may qualify for a discount (some telephone customers that don’t have any broadband service get a discount on these prices ) but it is if you don’t have broadband from any supplier not just BT what you are intending to get may or may not disqualify you from the discount, you would need to ask , this discount can’t be had ‘on line’ you need to call .
If you get a landline only service from BT , either immediately it’s provided or shortly after provision it will be Digital Voice so it will be IP telephony, you will effectively have broadband to run the telephone service , even if you don’t realise it , the bandwidth is needed is tiny , but if you are getting a 4G/5G mobile broadband then it’s weird not to use that for telephony and ditch the landline completely, if you do keep it for resilience, and it’s via local broadband , so you have a router in your home (that you don’t have to pay for ) then the service needs obviously mains power , so hopefully you won’t be assuming that it’s available in a power outage
